I sure hope the shake weight catches on. With workouts being as simple, cheap and efficient as that, there would be no fat people in the world in just a few months!
The shake weight is a product for people who want easy shortcuts to becoming fit, and don't have anywhere near the willpower needed to change their life styles and making real progress. If you buy the shake weight, I can almost guarantee you that you will put it away in a drawer or a box in the attic (or where ever) in a few months from now (at most), and never get anywhere near the shape of the people you see in the advertisement videos.
If you're gonna spend 30 bucks on something, get a dumbell set instead.